Question: Write the equation for a parabola with a focus at $(7,2)$ and a directrix at $y=-2$. $y=$
The strategy A parabola is defined as the set of all points that are the same distance away from a point (the focus) and a line (the directrix). Let $(x,y)$ be a point on the parabola. Then the distance between $(x,y)$ and the focus, $(7,2)$, is equal to the distance between $(x,y)$ and the directrix, $y=-2$. Once we find these distances, we can equate them in order to derive the equation of our parabola. Finding the distances from $(x,y)$ to the focus and the directrix The distance between $(x,y)$ and $(7,2)$ is $\sqrt{(x-7)^2+(y-2)^2}$. [How did we find that?] Similarly, the distance between $(x,y)$ and the line $y=-2$ is $\sqrt{(y+2)^2}$. [How did we know that?] Deriving the formula by equating the distances $\begin{aligned} \sqrt{(y+2)^2} &= \sqrt{(x-7)^2+(y-2)^2} \\\\ (y+2)^2 &= (x-7)^2+(y-2)^2 \\\\ {y^2}+4y{+4} &= (x-7)^2{+y^2}{-4y}{+4}\\\\ 4y{+4y}&=(x-7)^2 \\\\ 8y&=(x-7)^2 \\\\ y&=\dfrac{(x-7)^2}{8}\end{aligned}$ The answer The equation of our parabola is $y=\dfrac{(x-7)^2}{8}$. Here is the graph of our parabola. As expected, the distance between a point on the parabola, $(x,y)$, and the focus is the same as the distance between $(x,y)$ and the directrix. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${10}$ ${11}$ ${12}$ ${13}$ ${14}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ $y$ $x$ ${(x,y)}$